1. Field of the Invention
The present invention relates to a zoom lens, more particularly an image pickup apparatus including the zoom lens.
2. Description of the Related Art
Recently, image pickup apparatuses, such as video cameras, digital still cameras, broadcast cameras, and surveillance cameras, which include solid-state image pickup elements, such as CCD and CMOS, and cameras using silver-halide films have become smaller with increased functionality.
Accordingly, demand has increased for small, wide-field-angle, high-zoom-ratio (high-magnification-variation-ratio) zoom lenses having a small length and high resolution for use in imaging optical systems of the image pickup apparatuses.
To comply with such a demand, a positive-lead zoom lens has been proposed in which a lens unit having a positive refractive power is disposed at the object side.
An example of a positive-lead zoom lens is a four-unit zoom lens including a first lens unit having a positive refractive power, a second lens unit having a negative refractive power, a third lens unit having a positive refractive power, and a fourth lens unit having a positive refractive power in order from the object side to the image side.
A four-unit zoom lens having a simple structure in which the first lens unit is composed of a single negative lens and a single positive lens is described in U.S. Patent Application Publication No. 2007/0217024 and U.S. Pat. Nos. 7,151,638 and 7,167,320.
In addition, a four-unit zoom lens in which the first lens unit is composed of a cemented lens including a negative lens and a positive lens and in which the second lens unit is composed of three negative lenses and a single positive lens is described in U.S. Pat. Nos. 7,333,274 and 6,867,925.
In general, the size of the entire zoom lens can be reduced while maintaining a predetermined zoom ratio by increasing the refractive power of each lens unit included in the zoom lens and reducing the number of lenses.
However, in this type of zoom lens, as the refractive power of each lens surface increases, the lens thickness also increases and it becomes difficult to correct the aberrations.
In the above-described four-unit zoom lens, to achieve an increase in the zoom ratio and reduction in the size of the entire lens system while maintaining good optical performance, it is important to adequately set the refractive power of each lens unit, the lens structure, and the moving condition of each lens unit during zooming.
In particular, it is important to adequately set the moving condition of each lens unit during zooming, the refractive power (reciprocal of focal length) of the first lens unit, and the structure of each of the lenses included in the first lens unit.
It is extremely difficult to obtain a high zoom ratio and high optical performance over the entire zoom range unless the above-described factors are adequately set.